A heat-exchanger type ventilation system of certain kind starts icing and clogging up inside an airflow path in the heat exchanger where warm exhaust air flows from the indoors due to the influence of cold air being taken from outdoors in the adjoining airflow path, when an outdoor temperature becomes as low a temperature as −10° C. in winter season. Conventional heat-exchanger type ventilation systems generally use a structure equipped with means to block cold air to prevent clogging due to ice formation as disclosed in patent literature 1, for example. However, the reality has been that there are no heat-exchanger type ventilation systems practically serviceable in regions where outdoor temperatures became as low as −25° C.
Referring now to FIG. 15 to FIG. 17, description is provided hereafter of a heat-exchanger type ventilation system shown in patent literature 1. FIG. 15 is a sectional view of the conventional heat-exchanger type ventilation system showing a state of heat-exchanging operation. As shown in FIG. 15, the conventional heat-exchanger type ventilation system comprises main body 105 provided therein with motor 103 having air-exhaust fan 101 and air-intake fan 102 fixed thereto, and heat exchanger 104. The ventilation system further comprises exhaust airflow path 107 for guiding exhaust airflow 113 from exhaust port 106 opened in an indoor room to the outdoors, and intake airflow path 109 for guiding intake airflow 112 from the outdoors into the room through intake port 108, wherein exhaust airflow path 107 and intake airflow path 109 are formed to intersect with each other inside heat exchanger 104. There are also damper 110 disposed inside intake airflow path 109 at the upstream of heat exchanger 104 for blocking or not blocking intake airflow 112, and intake-air temperature detector 111 for detecting a temperature of the outdoor air taken inside main body 105.
FIG. 16 is a sectional view of the same heat-exchanger type ventilation system showing another mode of operation when the intake airflow is blocked. When intake-air temperature detector 111 detects a predetermined temperature set as a value just before freezing, or −3° C. for instance, damper 110 closes intake airflow path 109 completely to block intake airflow 112, as shown in FIG. 16, thereby allowing the ventilation system to operate only with exhaust airflow 113 to avoid icing. When the air temperature surrounding intake-air temperature detector 111 rises to a predetermined set value in the course of continuing the operation of exhausting only the warm indoor air, damper 110 opens intake airflow path 109 and makes the ventilation system to resume the heat exchanging operation.
FIG. 17 is a sectional view of the same heat-exchanger type ventilation system showing still another mode of operation, in which a part of the exhaust air is circulated while the intake airflow is being blocked. As a different mode from that of FIG. 16, a part of warm exhaust airflow 113 is diverted into intake airflow path 109 and let it pass through heat-exchanger intake airflow path 109a inside heat exchanger 104, as shown in FIG. 17 while damper 110 keeps blocking intake airflow 112, and to have this exhaust airflow 113 help melt ice if built-up.
As described, the conventional heat-exchanger type ventilation system operates only in the exhausting mode when intake airflow path 109 is blocked to prevent heat exchanger 104 from icing, such that it does not carry out the primary function of ventilating air while exchanging heat between the indoor air and the outdoor air at the same time. The conventional heat-exchanger type ventilation system also has a drawback that it produces a negative pressure in the room when it only vents the air through exhaust airflow path 107, which often causes cold drafts and dew condensation due to the air entering from outdoors through spaces in the building.    Patent Literature 1: Japanese Patent Unexamined Publication, No. 2005-233494